Spinal cord tracts and lesions
Spinal cord tracts are bundles of nerve fibers that carry sensory information up to the brain (ascending tracts) or motor commands down from the brain (descending tracts). Damage to specific tracts produces predictable patterns of sensory or motor loss.
The spinal cord's white matter is organized into tracts — longitudinal bundles of axons that each carry one kind of information in one direction. Ascending tracts deliver sensation from the body to the brain; descending tracts deliver motor commands from the brain to the muscles.
Three tracts dominate exam questions. The dorsal columns carry fine touch, vibration, and proprioception upward, crossing (decussating) in the medulla. The spinothalamic tract carries pain and temperature upward, crossing within a level or two of where its fibers enter the spinal cord. The lateral corticospinal tract carries voluntary motor commands downward, crossing in the medulla. Where each tract crosses determines whether a spinal injury produces symptoms on the same side (ipsilateral) or opposite side (contralateral) of the body.
Those crossing points make lesions readable like a map. In Brown-Séquard syndrome — hemisection of one side of the cord — the patient loses motor control, fine touch, and proprioception on the same side below the lesion (corticospinal tract and dorsal columns, which have not yet crossed) but loses pain and temperature on the opposite side (spinothalamic tract, which crossed near its entry). Other classic patterns include the dorsal column degeneration of vitamin B12 deficiency and the combined deficits of anterior spinal artery occlusion.
USMLE Step 1 tests spinal cord tracts and lesion localization heavily in neuroanatomy — expect vignettes asking which tract or level explains a deficit pattern. The MBLEx also covers nervous system organization, including the spinal cord's role in relaying sensory and motor signals, within its anatomy content.
Key takeaways
- Ascending tracts carry sensation to the brain; descending tracts carry motor commands to the body.
- Dorsal columns (fine touch, vibration, proprioception) and the corticospinal tract (motor) cross in the medulla; the spinothalamic tract (pain, temperature) crosses near its spinal entry level.
- Crossing points determine whether a lesion causes same-side or opposite-side deficits.
- Brown-Séquard syndrome pairs ipsilateral motor and proprioceptive loss with contralateral pain and temperature loss.
- USMLE Step 1 tests tract-based lesion localization; the MBLEx tests nervous system organization.